A quote each from some of my favourite and well-known writers and diarists. Their thoughts toward keeping a diary and what to include in it mirror my own from my previous post on the topic.
Joan Didion - "How much of it actually happened? Did any of it? Why do I keep a notebook at all?...The point of my keeping a notebook has never been, nor is it now, to have an accurate factual record of what I have been doing or thinking. That would be a different impulse entirely, an instinct for reality which I sometimes envy but do not possess."

Susan Sontag - "In the journal I do not just express myself more openly than I could to any person; I create myself. The journal is a vehicle for my sense of selfhood. It represents me as emotionally and spiritually independent. Therefore (alas) it does not simply record my actual, daily life but rather - in many cases - offers an alternative to it..."

Sylvia Plath - “I want to write because I have the urge to excel in one medium of translation and expression of life. I can't be satisfied with the colossal job of merely living. Oh, no, I must order life in sonnets and sestinas and provide a verbal reflector for my 60-watt lighted head.”

Anais Nin - “Ordinary life does not interest me. I seek only the high moments. I am in accord with the surrealists, searching for the marvelous.”

Virginia Woolf - “What sort of diary should I like mine to be? I should like it to resemble some deep old desk or capacious hold-all, in which one flings a mass of odds and ends without looking them through. I should like to come back, after a year or two, and find that the collection had sorted itself and refined itself and coalesced, as such deposits so mysteriously do, into a mould, transparent enough to reflect the light of our life, and yet steady, tranquil compounds with the aloofness of a work of art."